This invention relates to a filter holder useful in the filtration of intravenous fluids as they are supplied to patients undergoing intravenous therapy. More particularly, the invention relates to a holder for a microporous membrane filter adapted to separate gases and liquids while performing the filtration function.
Filter holder designs adapted to separate liquids from gases entrained therein while filtering the liquid are known. Specific designs for such filter holders are shown, for example, in U.S. Pat. No. 3,523,408, issued Aug. 11, 1970, to D. Rosenberg, U.S. Pat. No. 3,631,654, issued Jan. 4, 1972 to Riely et al and U.S. Pat. No. 3,854,907 to Rising.
It is necessary to remove air from intravenous fluid before the fluid enters the vein of a patient since bubbles which enter a vein can cause an air embolism with substantial danger of death to the patient. For this reason, filters which perform a liquid-gas separation have found particular application in the in-line filtration of intravenous fluids, i.e., filtration as the fluid is being supplied to the patient. Filtration to remove small particles from intravenous fluids as they are administered also reduces the incidence of phlebitis in patients undergoing intravenous therapy.
In general, in the prior constructions of gas-liquid separating filters, the liquid, containing gases entrained therein, is supplied to a chamber having two outlets, one of which is covered with filtration material which is wetted by the liquid while the other outlet is covered by material which is not wetted by the liquid. The typical small pore size of the wetted filter prevents gas from passing through said filter at the usual operating pressures. However, liquid will not pass through the wetted filter whenever a gas bubble contacts the filter surface. Thus, gas bubbles must be brought into contact with the non-wetted filter. Pressure within the chamber will tend to force gases entrained in the liquid through the non-wetted filter which thus acts as a gas vent.
Filter holders in which the non-wetting filter and the liquid filter oppose each other are shown, for example, in the cited Rosenberg patent. Rosenberg does not provide a means for facilitating the separation of gas and liquid within the filter unit in that there are no means provided for directing gas preferentially toward the hydrophobic filter and for directing the liquid toward the hydrophilic filter. Thus, there exists the possibility of gas accumulating at the hydrophilic filter which can lead to blockage of liquid through the hydrophilic filter. The cited Riely et al patent illustrates filter holder constructions in which the non-wetting and wetting filters are adjacent each other. The prior constructions, such as those illustrated in the foregoing patents, in general, provided a relatively large ratio of non-wetting to wetting filter, the ratio often approaching a value of 1 or even more. They also depend upon the buoyancy of the gas to cause the gas to contact the non-wetting filter. Additionally, the area of the wetted filter was not large in comparison to the filter holder. When, as is usual, microporous filters of the screen type are used in such holders, a relatively large wettable filter area is required to prevent filter clogging when in use. This is particularly true for holders to be used in intravenous therapy, since the available pressure to force liquid through the wetted filter is relatively small, i.e., a pressure corresponding to a liquid head of 3 to 4 feet. Also, filter holders made according to the prior art had relatively large internal volumes to permit entrained gases to move to the surface of the non-wetting filter without blocking the wetting filter, and hence were relatively bulky if they were made large enough to provide the desired wettable filter area; and because of their bulk and their general shape, they were unsuited for general use in intravenous therapy.
The cited Rising patent utilizes a filter holder having a hollow tubular support for the wetted filter. The outlet vent is located at one end of the filter rather than being on one side of the filter to minimize accidental blocking of the vent. The filter holder relies upon the buoyancy of air in liquid to effect air-liquid separation. While the apparatus is easier to manufacture and use as compared to the other available prior art apparatus, separation of air and liquid is dependent upon the position of the apparatus. It also has a relatively large internal volume to allow the gas bubbles to reach the vent filter.
In some forms of intravenous therapy, it is desirable for the liquid administration system to have a low internal volume to reduce the mixing of drugs given consecutively and to reduce the total amount of a drug given to the patient, as when the administration system is filled with heparin to prevent blood from clotting in the catheter between administrations of another drug. Accordingly, it would be desirable to provide a filtration device suitable for use in intravenous liquid administration which has low internal volume and yet prevents gas bubbles from occluding the liquid filter.